The present invention relates to grout for use with geothermal heat pump systems. In particular, the present invention relates to cement-sand grouts having improved thermal conductivity, as well as increased bond strength and decreased permeability.
Geothermal heat pump (GHP) systems for recovering energy from the ground are well known. The temperature of the Earth increases with increasing depth, to 400-1800.degree. F. at the base of the Earth's crust and to an estimated temperature of 6300-8100.degree. F. at the center of the Earth. Geothermal energy is present everywhere beneath the Earth's surface, but in order to be used as a source of energy, it must be accessible to drilling and, therefore, must be relatively close to the surface. Since a major cost in geothermal development is drilling and since the cost of drilling increases with increasing depth, shallow concentration of geothermal energy are preferred. The most conspicuous use of geothermal energy is the generation of electricity by providing high temperature water that is partly flashed to steam, and this steam is used to drive conventional turbine-generators.
GHP systems can use the ground as either a heat source or a heat sink for inexpensively heating and cooling residential m'd commercial buildings or for providing heated water. In some areas (particularly regions of low elevations), high concentration of geothermal energy are relatively close to the surface and provide an economical heat source. In other areas, the temperature of the ground is fairly constant at depths only a few yards below the surface. This constant temperature allows GHP systems to use the ground as either a heat source or a heat sink. When the ambient temperature is high, the GHP system is used as a heat sink to cool the fluid passing through the piping system so that when it returns to the surface it can be used for cooling. When the ambient temperature is low, the GHP system is used as a heat source to heat the fluid passing through the piping system so that when it returns to the surface it can be used for heating.
In general, a GHP system includes a pump, a piping system buried in the ground, an above ground heat transfer device and a heat transfer fluid that is circulated through the piping system by the pump. The ground is used either as a heat source to heat the circulating fluid or as a heat sink to cool the fluid. An important factors in determining the feasibility of a GHP system is the depth of wellbore, which affects the drilling costs, the cost of the pipe and the size of the pump. If the wellbore has to be drilled to too great a depth, a GHP system may not be a practical alternative energy source. One way to minimize the depth of the wellbore is to increase the heat transfer efficiency of the system.
An important aspect of all GHP systems is the grout that is used to secure the piping system in the ground. Vertically oriented ground heat exchangers for geothermal heat pumps (GHPs) require grout to be placed between the U-loop and the surrounding formation. When the U-loop for a GHP system is installed, a hole is drilled in the ground and the pipe is lowered into place. Grout is then poured around the piping to protect the pipes from the movement of the ground and from ground water. The grout provides a heat transfer medium and acts as a sealant to prevent contamination of the water supply. The thermal conductivity of the grout has a significant impact on the required depth of the wellbore and, hence, the installation costs. The more efficient the heat transfer between the fluid in the U-loop and the ground formation, the shorter the wellbore required to provide the desired heat transfer. A grout that does not efficiently conduct heat can act as an insulator and reduce the efficiency of the GRP system.
In the past, bentonite grouts have been used for GHP systems with mixed results. Although bentonite has good sealant properties, it has a relatively low thermal conductivity. Neat cement grouts have also been used with some degree of success, but high water to cementitious materials (w/c) ratios often create pores in the grout which cause a significant decrease in thermal conductivity. In addition, neat cement grouts with high w/c ratios are prone to shrinkage and do not form a satisfactory bond with the U-loop.
When choosing a grout formulation several factors have to be considered in order to improve thermal conductivity while meeting requirements for mixing and pumping with conventional equipment, permeability, shrinkage, bonding to the U-loop, durability, ease of handling, and economics. Other factors that must be considered are shrinkage, sulphate resistance, bleeding, environmental impact and coefficient of thermal expansion.
Various attempts have been made in the prior art to overcome problems caused by uneven curing, high well temperatures and heat transfer. U.S. Pat. No. 4,556,109 to Eilers discloses a curable slurry for cementing casing tubes in a wellbore. The slurry includes crushed coal and furfuryl alcohol, furfural, and/or a low molecular weight mono- or copolymer thereof U.S. Pat. No. 4,861,378 to Watanabe et al. discloses a cement additive containing superplasticizer, bentonite and inorganic strength-improving agents selected from calcium sulfates, silica fume.
U.S. Pat. No. 4,912,941 to Buichi discloses a method of extracting thermal energy from the earth's interior and uses a heat conducting grout made of a mixture of water and cement, along with one or more of a siliceous gel and finely divided metal powder, preferably silver and/or aluminum powder. BicMi also discloses the use of a special steel, asbestos-cement or a synthetic resin for thermal insulation of a pipe. U.S. Pat. No. 4,993,483 to Harris discloses a system for conditioning air in an enclosed space by transferring heat between the air and a heat exchange liquid. Harris uses sand or silica particles packed around tubes in the ground to thermally stabilize the tubes.
U.S. Pat. No. 5,038,580 to Hart discloses a heat pump system having a direct earth coupled underground heat exchanger. Hart teaches a grout material made of either cement alone or a mixed slurry of bentonite and water. U.S. Pat. No. 5,512,096 to Krause discloses a grouting material for sealing boreholes or other cavities that includes 90-99.99% water swellable clay and 0.01-10.0% gelling agent. The swellable clay can be bentonite and the preferred gelling agent is cement.
U.S. Pat. Nos. 5,590,715 and 5,758,724 to Amerman disclose the use of a bentonite clay mixture grout with systems that simultaneously introduce a loop of heat exchange pipe and a grout pipe into a wellbore. U.S. Pat. No. 5,706,888 to Ambs et al. discloses a volcanic clay or concrete grout for a heat pump circuit which operates to either cool or heat a space by transferring heat to and from an outside heat source/sink, such as the earth. U.S. Pat. No. 5,816,314 to Wiggs et al. discloses a geothermal heat exchange unit which can be placed in the ground and backfilled with fly-ash cement, concrete or the like to eliminate air gaps and increase thermal conductivity.
The grouts used in the prior art have not been completely satisfactory and there is still a need for a grout with improved heat transfer characteristics to improve the efficiency of GHP systems.